Electromagnetic radiation or ion flow detection tubes, such as photomultiplier tubes, image intensifier tubes and ion detection tubes, habitually include an input device to receive the electromagnetic radiation or the ion flow, and to emit in response a flow of electrons called primary electrons, an electron multiplier device to receive the said flow of primary electrons and to emit in response a flow of electrons called secondary electrons, and also an output device to receive the said flow of secondary electrons and to emit in response an output signal.
The electron multiplier includes at least one active structure intended to receive a flow of incident electrons, and to emit in response a flow of electrons called secondary electrons.
It may be of the reflection-mode electron multiplier type, where the active structure then has a single face for emission and reception of electrons. It may be formed, for example, from a microchannel wafer (GMC) or an assembly of dynodes.
The GMC is a high-gain electron multiplier which habitually takes the form of a fine plate, including a network of microchannels which traverse it from an upstream face facing towards the input device to an opposite downstream face facing towards the output device.
Two electrodes are present, one positioned on the upstream face of the GMC, the other on the downstream face. The GMC is thus subject to a difference of potential between its two faces, causing an electric field to be generated.
As is shown in FIG. 1A, which is a schematic and partial view of a microchannel 12 of a GMC, when a primary electron enters into a microchannel 12 and strikes its inner wall 14, secondary electrons are generated which, when they in their turn strike wall 14, generate further secondary electrons. The electrons are directed and accelerated by the electric field towards output aperture 13B of microchannel 12 located in downstream face 11B of the GMC.
The GMC's active structure thus essentially includes the substrate forming the said plate, where the substrate is made from lead glass. A lead reduction treatment is applied to optimise the secondary emission rate, and to make the inner wall of each microchannel semiconducting. The inner wall of each microchannel is then electrically connected to an external voltage source, which enables each microchannel to be supplied with the electrons intended to be emitted.
The GMC has a global gain G which depends on the elementary gain δi, or local secondary emission rate, at each step i of multiplication, and on multiplication number n in each microchannel, according to the following relationship:
                    G        =                                            ∏              i                        n                    ⁢                      δ            i                                              (        1        )            
In addition, the GMC has a signal-to-noise ratio which depends on noise factor F as F−1/2. And noise factor F also depends on elementary gain δi and on step number n of multiplication in each channel, as expressed by the following relationship:
                    F        =                  1          +                      1                          δ                              1                ⁢                                                                                                +                      1                                          δ                1                            ⁢                              δ                2                                              +                                    1                                                δ                  1                                ⁢                                  δ                  2                                ⁢                                  δ                                      3                    ⁢                                                                                                                                      ⁢                                                  ⁢            …                    +                      1                                          δ                1                            ⁢                              δ                2                            ⁢                                                          ⁢              …              ⁢                                                          ⁢                              δ                n                                                                        (        2        )            where δ1˜3-5 and δi>1˜2.
Increasing global gain G of the GMC may thus consist in increasing the length of each microchannel to increase the number of multiplication steps. However, as shown by relationship (2), this causes, de facto, an increase of noise factor F, and therefore a reduction of the signal-to-noise ratio. Increasing the GMC's global gain is thus accompanied by a degradation of the quality of the signal.
Another possibility to increase the GMC's global gain without degrading the signal-to-noise ratio may consist in using the known technique of chemical vapour deposition (CVD) to deposit a thin film of a material with a high secondary emission rate on the inner wall of the microchannels of the GMC. However, this would require that the substrate of the GMC is subjected to a temperature of over 800° C. over long periods, which is inconceivable given that the GMC's lead glass substrate cannot be subjected to higher than 430° C. without incurring a degradation of its structure and therefore of its performance.
Alternatively to the GMC, a reflection-mode electron multiplier device may be formed from an assembly of dynodes 2-1, 2-2, 2-3, etc., the configuration of which may be, in a known manner, of the venetian blind, box, linear focusing, circular cage, mesh, or foil type. A representation of these different categories of dynodes may be found in the work of Wernick & Aarsvold, 2004, Emission tomography: the fundamentals of PET and SPECT, Academic Press Inc.
The multiplier device has a global gain G which is written according to the same relationship as for a GMC, where δi is then the elementary gain of multiplication step i, in this case dynode i, and of the number n of dynodes.
The assembly of dynodes also has a rise time which depends directly on the elementary gain δi of each dynode and on the number n of dynodes, as expressed by the following relationship:
                    RT        ∝                              (                                          ∑                i                n                            ⁢                              σ                i                                      )                                1            /            2                                              (        3        )            where σi is the spread over time of the signal at dynode i. Rise time RT is conventionally defined as the time difference between 10% and 90% of the maximum value of a current pulse at the anode (output device) of the detection system, in response to a light pulse modelled by a delta function.
Increasing global gain G of the assembly of dynodes may thus consist in increasing the number n of dynodes, but causes de facto an increase of the rise time RT. Increasing the global gain of the assembly of dynodes is thus accompanied by a degradation of the multiplier's time resolution.